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Napier Grass (Pennisetum purpureum): An Emerging Biofuel Crop

Bhagwat W. Chavre*, Vitthal B. Sonawane

Department of Botany, Arts, Commerce and Science College, Nandgaon, Dist. Nashik, (M.S.), India

Abstract: Fossil fuels cannot be recommended due to their high cost, nonrenewable and pollution causing nature. It is predicted that, if present situation of fossil fuel exploitation will not change, Carbon dioxide emission in to atmosphere will increase by 50 to 250 %. Biofuels are now a day getting importance due to their low cost production, renewable and low pollution causing characters. The major sources of biofuel especially bioethanol are corn, sugarcane, sugar beet and wheat. But these are food crops which if used for the purpose of biofuel production could influence on food to population ratio. Napier grass is an efficient and effective solution to this problem as it produces lignocellulose biomass which can be used as a cheap and renewable source for biofuel production. Botanically, Napier grass is known as Pennisetum purpureum which belongs to grass family, Poaceae. In this article, author tried to throw light on Napier grass especially with its biofuel producing potential.

Keywords: Pennisetum purpureum, Napier grass, Biofuel, renewable, energy source

Received August 11, 2021; Revised September 18, 2021; Accepted September 26, 2021

Citation: Bhagwat W. Chavre, Vitthal B. Sonawane, Napier Grass (Pennisetum purpureum): An Emerging Biofuel Crop. Applied Ecology and Environmental Sciences, 9 (9). 811-813. January 2021. doi:10.12691/aees-9-9-5

1. Introduction

Fossil fuels raised up various disadvantages including its ever increasing prices, non-renewability and causes pollution by emitting wide range of carbon pollutants. Now a day, rising costs and the rapid consumption of the fossil fuels lead world to search for alternative energy source. Energy crops is the best and permanent solution which is renewable and comparatively cheap and releases less hazardous pollutants. Energy crops are again divided into two types, first generation and second generation crops. First generation energy crops generally are rich sources of starch and sugars and otherwise used as source of food. Their use for the purpose of biofuel production may lead to food crisis. Second generation energy crops however evolved to minimize this threat. These plants derived from lignocellulose material and agriculture waste such as grass, corn cob and wheat straw [1,2]. As compare to other crops, Napier grass has a potential to produce higher amount of biofuel and bioenergy per unit land area [3].

Napier grass or elephant grass (Pennisetum purpureum) is a newly evolving food crop considered suitable for biofuel production which having production capacity approximately 87 tons per hectare. It can be harvested five to six times in a year due its rapid growth [4]. Another important parameter associated with the grass are high productivity, low production cost, can grow on infertile land and whole plant utilization [1]. The Napier grass has higher tolerance against diseases and pests and shows vigorous growth even at stress conditions [5]. It is commonly used as animal feed in Malaysia and also as a food source in Nigeria [6]. Napier grass is a warm season perennial grass native of Africa adapted well to tropical and subtropical climates [7].

Biofuels especially bioethanol and biodiesel originated from renewable energy sources like Napier grass are gaining importance as they can be used to minimize load on fossil fuels. Fossil fuels have many damaging effects on environment such as greenhouse effect. It is estimated that carbon dioxide emission in the world can be reach up to 50-250 % by the year 2050, in which transportation sector has highest share [8]. Biofuels derived from lignocellulosic biomass such as Napier grass have several other benefits to society such as facilitating local economy development and stimulation, reduction in air pollution caused due to burning of biomass in fields. It also helps to minimize land and water pollution caused due to biomass rotting in fields. Biofuels helpful to bring energy security for countries dependent on imported oil. Biofuel production can produce job opportunities to engineers, fermentation specialists, and scientists [9].

Napier grass is highly suitable material for biogas production which contains the composition of carbon dioxide (30.10 %), methane (63.50%) and 5 ppm of H₂S gas. It is observed that NaOH pretreated sample produces high yield of biogas than untreated raw material [10].

There are several challenges for use of Napier grass for the production of biofuels. These includes food-fuel conflicts, negative energy balance, large quantity of water consumption, shortage of feedstock etc. But, workers reported that, ethanol contribute to energy and environmental goals [11]. Industrial production of bioethanol from lignocellulosic material such as Napier grass can be achieved by three different steps including pretreatment, hydrolysis and fermentation [1,8].

2. Botanical Information

Napier grass is abundantly seen in Nigeria as an indigenous grass. Napier grass sometimes known as Elephant grass or Uganda grass is a monocot belongs to family Poaceae and genus Pennisetum. Pennisetum is a diverse genus consisting of approximately 10,000 species with basic chromosome number 7, 8 or 9 ploidy ranging from diploid to octaploid [2]. Genetically, it is a tetraploid (2n=4x=28) coupled with 4.60 pg genomic DNA content [12]. Napier grass is rhizomatous perennial plant due to its profuse root system holds soil tightly hence controls soil erosion. The rhizomes are short and creeping and nodes of which develop fine roots and culms. The grass forms clumps and grows upward, profusely branched with thick cane-like stem. Pennisetum purpureum produces long, tapering and stiff leaves. The leaf edges have bristle-like ridges. The upper surface of leaf is covered with stiff hairs and leaf sheaths are also very hairy [13].

The plant bears panicles as a reproductive part which is characterized by purple in colour, sessile fascicles and sparsely plumose bristles. The stigma of plant matures before anthers which facilitates cross pollination. Seeds of the plant are very small and are dispersed by wind [13]. The grass grows very rapidly and can be harvested 4-5 times a year. Externally, plant looks like sugarcane [8]. Napier grass is considered as potential energy crop in Thailand, the Philippines, China, Australia and United states [14].

3. Chemical Contents of Napier Grass

Chemically, Napier grass contains high amount of flavonoids and other nutrients such as minerals, vitamins and dietary proteins [6]. Napier grass also contains, Moisture (75 - 77 %), Volatile matter (80 - 82 %), Ash (1.75-3.18 %), Carbon (44-48 %), hydrogen (6.00 %), nitrogen (1-2 %), oxygen (43-45 %) and Sulphur (0.06-0.32 %), Cellulose (38 %), hemicellulose (19-20 %), Lignin (27 %) [10,15].

4. Biofuel Production from Napier Grass

Napier grass after attaining full maturity can be processed for the production of biofuel in the form of ethanol. All parts of grass such as leaves, stalk and trunk can be used to produce bioethanol. Napier grass can be converted to liquid as well as gas fuels. In liquid forms it can be converted either into bioethanol or bio-oil. Bio-oil has less heating value as compare to bioethanol; hence it is more suitable liquid fuel. Bio-oil can be produced by the process of pyrolysis [16].

Complete conversion of Napier grass in to bioethanol requires 94 hours. The operations include pretreatment, hydrolysis, fermentation and finally product separation.

Pretreatment involves processes like harvesting, cleaning, drying and grinding of biomass. Pretreatment of biomass has several applications and methods. Physical pretreatment includes grinding, milling and extrusion of biomass at elevated temperature. In chemical pretreatment various chemicals such as, sulphuric acid, ammonia, NaOH, Hydrogen peroxide, cellulose enzyme, acetic acid etc can be used for the purpose of soaking, wet oxidation, steam explosion, ozonolysis etc. [9,17,18]. This process serves to reduce the crystalline nature of cellulose, increase porosity of biomass and helps in the desired fractionation. Proper pretreatment of biomass eases and improves hydrolysis and fermentation. Proximate composition analysis is carried out through standard procedures. Moisture content, fat or lipid content, ash content, crude protein content, fat of lipid content, ash content etc. are some of the important parameters to be analyzed [8]. In another method, the lignocellulosic material is also pretreated with 1.0, 2.0 and 3.0 % sodium hydroxide solution with 10 % total solid content. It is autoclaved at 121°C for 60 minute. Sometimes, the reaction mixture was subjected to alkali hydrolysis at different reaction times of 24, 48 and 72 hours. Thermal hydrolysis can also be done through steam as well as by the application of hot water. After that, mixture was cooled at room temperature and residue is washed with water to adjust pH to neutral. Filtration is carried out to separate solid residue which is then oven dries and stored in plastic bag at room temperature for the hydrolysis [1,10,19].

Biomass is hydrolyzed by various enzymes secreted by microorganisms which converts biomass into sugars. The enzymes for degradation of biomass can be used singly or in cocktail (40-50 enzymes in mixture to break different type of biomass). Biomass is hydrolyzed by acidic hydrolysis using 0.5 M dilute sulphuric acid. In 5 g biomass 100 ml, 0.5 M dilute H₂SO4 is added which subsequently heated at 155°C temperature for 30 minute. Continuous shaking is carried out during heating. The same procedure can be repeated for altering the biomass concentrations 10g, 15g and 20g respectively. To adjust pH (5.0) 10M NaOH solution is added dropwise [8]. Sometimes, cellulose hydrolyzing enzymes such as iKnowzyme, Acid Cellulase etc. are used for hydrolysis [1].

Fermentation can be done by preparing 100ml fermentation media by suing 10g dextrose sugar, 0.2 g yeast extract and 1g serving as nutrients. The media is allowed to fermentation using 0.5g of Sacchromyces cerevisae at 35°C for a period of 1 to 5 days and subjected to distillation process at 78.3°C [8]. However, S. cerevisiae is a non-thermotolerant yeast is impaired at high growth temperatures. Therefore, in tropical countries bioreactors should be cooled before utilization of S. cerevisiae which increase cost of ethanol production. Therefore, some workers recommend another type of thermotolerant yeast, Kluyveromyces marxianus, which permit decrease of cooling costs [17].

Production of solid fuels from Napier grass is highly efficient and also cost effective as compare to liquid and gas fuels. However, solid fuels produce more greenhouse gases than liquid and gaseous fuels. So, many world countries adopted policies for the production and utilization of biofuels in the form liquids and gas. It led to considerable decrease in the global warming in those countries [16]. Scientists [19] adopted two methods for the production of biogas in the form of methane from Napier grass. These includes briquetting and anaerobic digestion. Briquetting is a mechanical compaction process for increasing the density of bulky material. By using briquetting, machines the product briquette is formed which is a block of flammable matter can be used as a fuel. Second method used by them is the production of biogas by the microbial degradation of biomass in absence of oxygen. It includes sub-processes like hydrolysis, acidogenesis, acetogenesis and methanogenesis.

Pyrolysis is an important thermochemical techniques used for the conversion of biomass into solid (bio-char), liquid (bio-oil) and gaseous products under an inert environment [15]. Bio-oil produced by the pyrolysis process from Napier grass composes different organic and hydrocarbon compounds. Hydrocarbon compounds of bio-oil makes it combustible whereas, organic compounds such as organic acids and heterocyclic substances makes bio-oil more viscous and easily polymerized. The chemical compounds present in bio-oil have been reported by some scientists [16]. It is reported that increasing nitrogen flow by 20 to 30 ml/min and reaction temperature between 450 and 600°C increased the bio-oil yield [15,20].

5. Conclusion

World today facing many issues associated with fossil fuels which includes their ever-growing costs, pollution generation, nonrenewable nature etc. Biofuels originated from the biomass of various crops can be an efficient alternative source of energy. Napier grass escaped from food-fuel crisis due to its limited food value. Qualities like extensive and rapid growth habit, renewability, growing capacity in various soils, resistance to biological and environmental stresses and production of large biomass made this plant as a popular biomass source for energy generation. Biofuels in the form of liquid, solid and gas can be produced from Napier grass by adopting various techniques of fermentation, pyrolysis, anaerobic digestion and many others. It is emerging as a efficient and chief candidate for biofuels formation in future.

References

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